Ethanol is rapidly becoming a major hydrogen-rich liquid transport fuel around the world. Worldwide consumption of ethanol in 2002 was an estimated 10.8 billion gallons. The global market for the fuel ethanol industry has also been predicted to grow sharply in future, due to an increased interest in ethanol in Europe, Japan, the USA and several developing nations.
For example, in the USA, ethanol is used to produce E10, a 10% mixture of ethanol in gasoline. In E10 blends the ethanol component acts as an oxygenating agent, improving the efficiency of combustion and reducing the production of air pollutants. In Brazil, ethanol satisfies approximately 30% of the transport fuel demand, as both an oxygenating agent blended in gasoline, or as a pure fuel in its own right. Also, in Europe, environmental concerns surrounding the consequences of Green House Gas (GHG) emissions have been the stimulus for the European Union (EU) to set member nations a mandated target for the consumption of sustainable transport fuels such as biomass derived ethanol.
The vast majority of fuel ethanol is produced via traditional yeast-based fermentation processes that use crop derived carbohydrates, such as sucrose extracted from sugarcane or starch extracted from grain crops, as the main carbon source. However, the cost of these carbohydrate feed stocks is influenced by their value as human food or animal feed, while the cultivation of starch or sucrose-producing crops for ethanol production is not economically sustainable in all geographies. Therefore, it is of interest to develop technologies to convert lower cost and/or more abundant carbon resources into fuel ethanol.
CO is a major free energy-rich by-product of the incomplete combustion of organic materials such as coal or oil and oil derived products. For example, the steel industry in Australia is reported to produce and release into the atmosphere over 500,000 tonnes of CO annually.
It has long been recognised that catalytic processes may be used to convert gases consisting primarily of CO and/or CO and hydrogen (H2) into a variety of fuels and chemicals. However, micro-organisms may also be used to convert these gases into fuels and chemicals. These biological processes, although generally slower than chemical reactions, have several advantages over catalytic processes, including higher specificity, higher yields, lower energy costs and greater resistance to poisoning.
The ability of micro-organisms to grow on CO as their sole carbon source was first discovered in 1903. This was later determined to be a property of organisms that use the acetyl coenzyme A (acetyl CoA) biochemical pathway of autotrophic growth (also known as the Woods-Ljungdahl pathway and the carbon monoxide dehydrogenase/acetyl CoA synthase (CODH/ACS) pathway). A large number of anaerobic organisms including carboxydotrophic, photosynthetic, methanogenic and acetogenic organisms have been shown to metabolize CO to various end products, namely CO2, H2, methane, n-butanol, acetate and ethanol. While using CO as the sole carbon source all such organisms produce at least two of these end products.
Anaerobic bacteria, such as those from the genus Clostridium, have been demonstrated to produce ethanol from CO, CO2 and H2 via the acetyl CoA biochemical pathway. For example, various strains of Clostridium ljungdahlii that produce ethanol from gases are described in WO 00/68407, EP 117309, U.S. Pat. Nos. 5,173,429, 5,593,886, and 6,368,819, WO 98/00558 and WO 02/08438. The bacterium Clostridium autoethanogenum sp is also known to produce ethanol from gases (Aribini et al, Archives of Microbiology 161, pp 345-351 (1994)).
However, ethanol production by micro-organisms by fermentation of gases is always associated with co-production of acetate and/or acetic acid. As some of the available carbon is converted into acetate/acetic acid rather than ethanol, the efficiency of production of ethanol using such fermentation processes may be less than desirable. Also, unless the acetate/acetic acid by-product can be used for some other purpose, it may pose a waste disposal problem. Acetate/acetic acid is converted to methane by micro-organisms and therefore has the potential to contribute to Green House Gas emissions.
The importance of controlling parameters of the liquid nutrient medium used for culturing bacteria or micro-organisms within a bioreactor used for fermentation has been recognised in the art. NZ 556615, filed 18 Jul. 2007 and incorporated herein by reference, describes, in particular, manipulation of the pH and the redox potential of such a liquid nutrient medium. For example, in the culture of anaerobic acetogenic bacteria, by elevating the pH of the culture to above about 5.7 while maintaining the redox potential of the culture at a low level (−400 mV or below), the bacteria convert acetate produced as a by-product of fermentation to ethanol at a much higher rate than under lower pH conditions. NZ 556615 further recognises that different pH levels and redox potentials may be used to optimise conditions depending on the primary role the bacteria are performing (i.e., growing, producing ethanol from acetate and a gaseous CO-containing substrate, or producing ethanol from a gaseous containing substrate).
U.S. Pat. No. 7,078,201 and WO 02/08438 also describe improving fermentation processes for producing ethanol by varying conditions (e.g. pH and redox potential) of the liquid nutrient medium in which the fermentation is performed.
The pH of the liquid nutrient medium may be adjusted by adding one or more pH adjusting agents or buffers to the medium. For example, bases such as NaOH and acids such as sulphuric acid may be used to increase or decrease the pH as required. The redox potential may be adjusted by adding one or more reducing agents (e.g. methyl viologen) or oxidising agents.
Similar processes may be used to produce other alcohols, such as butanol, as would be apparent to one of skill in the art.
Regardless of the source used to feed the fermentation reaction, problems can occur when there are breaks in the supply. More particularly, such interruptions can be detrimental to the efficiency of the micro-organisms used in the reaction, and in some cases, can be harmful thereto. For example, where CO gas in an industrial waste gas stream is used in fermentation reactions to produce acids/alcohols, there may be times when the stream is not produced. During such times, the micro-organisms used in the reaction may go into hibernation. When the stream is available again, there may then be a lag before the micro-organisms are fully productive at performing the desired reaction.
Sulphur sources such as cysteine and/or sulfide are also used to attain desirable ORP (oxidation-reduction potential) of the anaerobic fermentation broth prior to inoculation. However, such reducing agents are slow and have limited reducing power. Furthermore, when these sulphur containing compounds are used to reduce ORP of a fermentation media, they are oxidised themselves. For example, cysteine is oxidised to the dimer cystine. It is considered that the reduced form of these compounds is substantially more bioavailable as a sulphur source for consumption by a microbial culture than the oxidised form. As such, when a sulphur source is used to lower the ORP of a fermentation reaction, the actual concentration of sulphur available to the microbial culture will decrease. Accordingly, identification of an improved or alternative reducing agent for use with anaerobic fermentation systems using carbon monoxide containing gases as a feedstock, is a key component in ensuring high alcohol production rates and low process operating costs.
Along with main nutrients such as nitrogen and phosphorus, sulphur plays an important role in the fermentation of the anaerobe C. autoethanogenum. Sulphur is essential for the microbe and is needed for a range of compounds and enzymes that allow C. autoethanogenum fermentation of CO into acetic acid, ethanol and butanediol and to generate ATP for growth of biomass. Sulphur is part of a class of biological compounds called ferredoxins and is an integral part of many of the Wood-Ljungdahl enzymes that fix gaseous CO into acetyl-Co-A. Generally the most reduced form of sulphur is assimilated into functional proteins. The microbe can take up H2S directly or in the form of the hydrogensulfide ion and assimilate it into the desired proteins. Many of the microbes sulphur containing enzymes also contain transition metal ions such as Fe2+, Zn2+, Co2+ and Mn2+. As sulphides of these metals have very low solubility products at pH values around neutral, free H2S and or free transition metals are usually scarce in such habitats, because the majority of the metal ions will be bound into insoluble metal sulphides and are therefore not accessible to the microbes.
It is an object of the present invention to provide a system and/or a process that goes at least some way towards overcoming the above disadvantages, or at least to provide the public with a useful choice.